1. Field of the Invention
The present invention relates to a method of directly softening rolled machine structural steels, particularly those which are to be worked into bolts, or the like shapes by cold forging.
2. Prior Art
Heretofore, when producing machine parts from machine structural steels by cold forging, the steels have been customarily subjected to spheroidization annealing of cementite prior to cold forging, with an intention of softening them, or reducing their resistance to deformation. Since this softening treatment takes as long as 10-20 hours, it has long been desired to develop a soft rolled steel that does not need any such spheroidization annealing, from the viewpoint of achieving improved productivity or reduced energy consumption.
While various proposals have been made in an attempt to attain this object, for instance, "Tetsu to Hagane (Iron and Steel)", 70, 5, 236, 1984 proposes, on the premise, that such medium carbon machine structural steels specified in the currently effective JIS (e.g. S45C and SCM435) are to be used and that the steel should be softened by rolling at low temperatures near 675.degree. C. followed by isothermal holding of them at a specified temperature. This method, however, is not considered a satisfactory solution because such rolling in the low temperature range will cause surface defects in wires or reduced durability of working rolls.
There exist much patents literature proposing techniques for elimination of spheroidization annealing. Laid-Open Japanese Patent Publication No. 107416/1983 shows a softening method wherein a steel is rough-rolled to achieve a reduction in thickness of 30% or more at a temperature not lower than 1,000.degree. C., then finish-rolled to achieve further reduction in thickness of 50% or more in the temperature range of from 750.degree. to 1,000.degree. C. and, thereafter, is cooled to the completion of transformation at a cooling rate not faster thatn 1.degree. C./sec. Lain-Open Japanese Patent Publication No. 13024/1984 discloses a spheroidizing technique of carbides wherein a steel is finish-rolled to achieve a reduction in thickness of 30% or more in a temperature range between a point not higher than the Ar.sub.1 point and one not lower than the Ar.sub.1 point minus 50.degree. C. and then the rolled steel is reheated in the temperature range of Ac.sub.1 -Ac.sub.3. Laid-Open Japanese Patent Publication Nos. 126720 and 126721/1984 disclose a carbide spheroidizing technique, wherein a steel is finish-rolled to achieve a reduction in thickness of 80% or more in a temperature range between a value not higher than the Ar.sub.1 point and the point not lower than the Ar.sub.1 minus 50.degree. C. and the subsequent rolling operation is then finished either at a temperature in the range of Ac.sub.1 -Ac.sub.3 by using the heat resulting from rolling, or the rolled steel is immediately cooled to produce the structure of spheroidized carbide. Laid-Open Japanese Patent Publication Nos. 136421, 136422 and 136423/1984 propose a carbide spheroidizing technique wherein a steel is finish-rolled to achieve a reduction in thickness of 10% or more in a temperature range between a value not higher than Ar.sub.1 and one not lower than the Ar.sub.1 point minus 200.degree. C., then the rolled steel is heated to a temperature in the range defined by a value not higher than the Ac.sub.3 point but one not lower than the Ac.sub.1 point minus 100.degree. C. using the heat resulting from rolling, and the steel then is cooled from that temperature down to 500.degree. C. at a cooling rate not faster than 100.degree. C./sec, alternatively the heated steel is either held for 7 minutes or longer in the temperature range of not higher than the Ac.sub.1 point but not lower than 500.degree. C., or the steel is subjected to repeated cycles of controlled rolling at a temperature not higher than Ac.sub.3 but not lower than the Ac.sub.1 point, both aiming at spheroidizing of cementite particles. Subsequently the steel is rolled to achieve a reduction in thickness of 15% or more, and heated to a temperature not lower than the Ac.sub.1 point but not higher than the Ac.sub.3 point by utilizing the heat of deformation. Either these techniques, however, involve the problems of increased surface defects and reduced durability of working rolls, since these methods obtain rolled soft steels by restricting the condition of hot rolling by means of effecting finish rolling at a lower temperature, in comparison with ordinary hot rolling which is usually finished at about 1,000.degree. C.
As is well known, for example, Laid-Open Japanese Patent Publication No. 136421/1984 mentioned above, discloses that micro structures of steels as rolled vary somewhat depending on the kind of steel: steels of low hardenability have either pearlite or ferrite-pearlite structure, while alloy steels having high hardenability have bainite structure. Therefore, in order to reduce the strength of rolled steel, it is necessary to prevent the formation of bainite having high strength, to produce ferrite-pearlite structure and further to reduce the strength of the pearlite that accounts for the major part of the steel structure. In view of the generally established theory that the strength of pearlite is inversely proportional to the lamellar spacing of the cementite in the pearlite, the lamellar spacing must be widened if one wants to decrease the pearlite strength.
However, the lamellar spacing of cementite in the pearlite is solely determined by the temperature at which pearlite transformation from austenite takes place, and the higher the transformation point is, the more coarse the lamellar spacing of the cementite becomes. This means that in order to soften a rolled steel, transformation to pearlite must be done at high temperatures by either cooling the as-rolled steel slowly or by holding the as-rolled steel immediately after rolling at the highest possible temperature in the range wherein such pearlite transformation takes place. However, the rate at which the pearlite transformation proceeds decreases with increasing temperatures, and thus as excessively long period of time is required before the transformation is completed if the steel is transformed at higher temperatures. The problem is that whichever of the two softening methods is to be employed, the equipment or production line available today imposes inherent limitations with regard to the rate of slow cooling or to the period for which the rolled steel is maintained at the highest temperature that is practically possible.
The present inventors analyzed the aforementioned findings on the prior art and made various studies on the factors that would govern the properties in the strength of rolled machine structural steels. As a result, the inventors found that the two objectives, i.e. preventing formation of bainits having high strength together with an increase in the lamellar spacing of the cementite in pearlite, which is a very effective means for softening or reducing the strength of the medium carbon steel under conventional conditions of hot rolling and at the same time completing the pearlite transformation at a higher temperature in a shorter period of time which is also crucial to the purpose of softening the rolled steel, can be attained simultaneously by substituting Cr for a part of the Mn in the prior art steel and by employing appropriate conditions for cooling or holding the hot rolled steel after hot rolling. The present inventors have proposed a method which was accomplished on the basis of these findings and filed a patent application as Japanese Patent Application No. 13891/1985 filed on Jan. 28, 1985 and has been laid open on Aug. 6, 1986 as Laid-Open Japanese Patent Publication No. 174322/1986, and this invention corresponds to U.S. patent application Ser. No. 821,550. Although this method is very effective with respect to softening the rolled low alloy steels having low hardenability, there yet remains various rooms for improvement with respect to the softening of rolled alloy steels having a high extent of hardenability such as SCr or SCM steel.